Methods and agents for improving paper printability and strength

ABSTRACT

Compositions containing hydrophilic polyacrylamide and hydrophobic surface size agents and the methods for using such compositions in paper and board to improve printability, sizing and strength are provided in the instant invention.

This application claims the benefit of the filing date of ProvisionalApplication Ser. No. 60/003,273 filed Aug. 25, 1995.

FIELD OF THE INVENTION

This invention relates to compositions and methods for improving theprintability and/or strength of various types of paper and board. Morespecifically, this invention relates to mixtures of hydrophobic surfacesizes and hydrophilic polyacrylamide which, when added to paper orboard, improve print quality, sizing, as well as surface strength of thepaper or board.

BACKGROUND OF THE INVENTION

Sizing agents have gained wide acceptance in the paper industry formodifying paper to improve its resistance to penetration by liquids,generally water, and to improve various aspects of printability. Forexample, U.S. Pat. No. 5,122,568 and 5,139,614 teach the use of styreneacrylic copolymers for enhancing sizing, inkprint contrast, increasingink penetration times and reducing feathering. U.S. Pat. No. 3,562,102discloses amine reaction products of alkyl glycidyl mixed esters whichare used in the preparation of sized cellulosic paper substrates, curedfilms and solid resinous potting compositions.

Efforts have been made to develop improved sizing agents. In U.S. Pat.No. 4,855,343 acrylonitrile acrylate monomers are copolymerized byemulsion polymerization in the presence of degraded starch and aperoxide-containing initiator to provide improved paper sizing agents.U.S. Pat. No. 5,362,573 describes the use of, titanium, hafnium andzirconium salts such as ammonium zirconium carbonate in conjunction withsurface sizing compound containing water-soluble hydroxylated polymer toimprove sizing efficiency.

There are many different sizing agents used in papermaking, such asrosin soap, starch, sodium silicate, amine reaction products of alkylglycidyl mixed esters, such as that described in U.S. Pat. No.3,562,102, for example, and sizing can be accomplished by eitherinternal sizing processes, which typically involve wet end addition, orsurface sizing processes, which typically involve addition at the sizepress. Surface treatment at the size press typically with sizing agentsalters the paper web primarily by imparting an increased degree ofhydrophobicity. This sizing effect, in turn, generates higher inkpenetration time values, reduced lateral spread of printing inks, aswell as improved imaging and contrast with various printingtechnologies. Thus, hydrophobic surface size agents improve printabilityprimarily by decreasing paper sheet absorbtivity and enhancing surfaceresistance to liquid (ink) penetration.

Paper printability and other qualities have also been improved throughthe use of coated paper and various ink jet recording sheets. In U.S.Pat. No. 4,442,172, an ink jet recording sheet made by coating awater-soluble polymer onto the surface of a base sheet containingsynthetic silicate, wet strength additive and glass fiber is describedas having superior ink absorbency and high recording density withoutfeathering. Combinations of alkyl ketene dimer internal sizing agent,precipitated calcium carbonate and certain cationic synthetic fixingagents have also been incorporated into ink jet recording base layers,see, U.S. Pat. No. 5,126,010. Ink jet recording paper described in JP06143799 is prepared by applying a surface size treatment of highmolecular weight adhesive and styrene-maleic anhydride copolymer to bothsides of base paper and disposing an ink-receiving coating containing awhite pigment and adhesive on one side of the paper. Ink jet recordingtransfer sheets containing calcium carbonate as a filler and alkylketene dimer or alkenyl succinic anhydride as sizing agents aredescribed in JP 06008617. Auxiliary agents such as starch,polyacrylamide and polyvinylalcohol may be included in the transfersheet. J56109783-A describes paper for ink jet recording obtained byapplying a hydrophilic polymer, e.g., oxidized starch or polyvinylalcohol, to raw paper of specified water absorption degree to providepaper with balanced ink drying properties and blotting resistance forink-jet printing. And U.S. Pat. No. 4,900,620 describes ink jetrecording sheets having good ink absorption, resolution and colorbrightness which are made of wood pulp and precipitated calciumcarbonate substrate coated with a layer of white pigment. Ink jetrecording sheets composed of a support and a cation-modified acicular orfibrous colloidal silica-containing ink-receiving layer are described inU.S. Pat. No. 5,372,884; various polymers may be included in the inkreceiving layer for improving ink drying properties, film-formingproperties and image sharpness.

In U.S. Pat. No. 4,294,704 paper and board is coated with compositionscontaining aqueous latex binders of synthetic polymer such asstyrene-butadiene copolymers, carboxyl styrene-butadiene copolymers,vinyl polyacetates, carboxyl vinyl polyacetates, alkyl acrylate-vinylacetate copolymers and carboxyl alkyl acrylate-vinyl acetate copolymersin order to provide improved dry and wet tear resistance. In U.S. Pat.No. 4,439,496 a water-resistant photographic substrate containing analkylketene dimer, a cationic polyacrylamide and an anionicpolyacrylamide is coated with a water resistant polyolefin.

JP05173287 discloses base paper for use in photographic printing paperwhich is made by adding a cationic polyacrylamide to pulp slurry, thenadding a sizing agent which has an effect in the weakly acidic to weaklyalkaline pH region, e.g., high organic ketene dimers, and finally addingan anionic polyacrylamide; the base paper is described as being usefulin inhibiting the permeation of developing liquids from the cut endsurfaces of the photographic printing paper. Another ink jet recordingsheet is described in EP 600245.

As new printing technologies continue to emerge and gain acceptance inthe market place, the need for developing high performance papersurfaces having improved printability characteristics increases. Despitethe benefits obtained from using coated paper or ink jet recordingsheets, these materials have certain disadvantages, for example, theyrequire two separate layers and therefore their manufacture involvesnumerous paper processing steps and special converting operations.Furthermore, many of the existing coated paper sheets exhibit inadequatecolor image, color resolution and color bleed. Moreover, as paperproducts increasingly use multiple colored inks, paper exhibitingcontrolled ink absorption, optimal color density and reduced colorbleed, and with reduced feathering and wicking has been particularlydifficult to make. It is desirable, in many instances, to use singlesheets of paper having superior color absorption rates, color densityand resolution with reduced color bleed and feathering.

Consequently, a need, unsatisfied by existing technology, has developedfor a paper additive that provides the above-described benefits inprintability and sizing yet also exhibits superior dry strength.

Existing dry strength additives have not met this need.N-(chloroalkoxymethyl) acrylamides and quaternized derivatives thereofare dry strength additives described in U.S. Pat. No. 3,980,800.Canadian patent 1101158 describes improved dry strength propertiesobtained by admixing a fibre suspension with quaternized acrylamidepolymers. The addition ofacetone-triethylenetetramine-diethylenetriamine-formaldehyde copolymerand acrylamide-maleic anhydride copolymer to cellulose pulp is describedin U.S. Pat. No. 4,036,682 as giving increased strength properties.Polyvinyl alcohol and starch have been applied to paper to improve sheetsurface characteristics which may included surface strength, andporosity smoothness. In JP 57139599 and JP 139598 anionic polyacrylamideis added at the wet end and a sizing agent selected from alkyl ketenedimer, cationic sizing agent and substituted cyclic dicarboxylicanhydride is used along with aluminum salt to manufacture neutral paperpurportedly having good wet and dry tenacity.

Various printing technologies continue to require paper surfaces withhigh integrity and resistance to the abrasive effects of the printingprocesses. Fiber picking, linting and filler dusting all contribute todecreased runnability of paper on impact printing processes. Theretherefore exists a need for surface treatment agents which overcomethese surface defects.

It has now been discovered that a generally hydrophobic surface sizeadditive and a hydrophilic dry strength agent, preferablypolyacrylamide, may be combined in a single composition to provide, whenadded to paper or board sheet surfaces at the size press,synergistically improved sizing and strength as well as a wide range ofimproved paper printability properties. It was surprising to find that ahydrophilic agent such as polyacrylamide could be combined with ahydrophobic surface size as a pre-mix and that improved strength, sizingand printability properties resulted from use of that pre-mix in paperor board including paperboard. This was particularly unexpected giventhat hydrophobic surface size agents, in theory, increase the degree ofhydrophobicity of the sheet and thus decrease paper absorptivity whilehydrophilic materials such as polyacrylamide increase the degree ofhydrogen boding in the paper sheet and maintain high sheet absorbitivityof liquid. Paper or board made using the compositions of the instantinvention exhibit sizing properties which are superior to the use of thehydrophobic surface size agent alone. Thus, by admixing polyacrylamidewith a surface sizing agent the sizing function is unexpectedlyimproved. A further benefit provided by the compositions of the instantinvention is synergistically improved strength properties, such asinternal bond strength and wax pick. In addition, paper and boardcomprising the surface size polyacrylamide mixtures of the instantinvention exhibit synergistic improvements in print quality, e.g.,controlled ink absorbency, ink drying time and color optical density;sharper half-tone dots are produced with maximized light scatter forimproved print contrast. The products of the instant invention impartsubstantial resistance to penetration of ink and aqueous liquids to saidpaper, provide processing flexibility by allowing the papermaker tobalance the size requirements between internal and surface sizing,increase the contact angle and surface strength, maintain the sizinglevel over time and impact the coefficient of friction. Other benefitsrelating to printability such as improved image resolution and reducedcolor bleed, bleed through, feathering, wicking, picking, linting anddusting are also achieved by the instant invention. The superior imageresolution and bleed through properties provided by the compositions ofthe instant invention are particularly desirable for use in connectionwith modern printing technologies which use a variety of different colorinks. Advantageously, the above benefits are provided by a single sheetof uncoated paper or board without having to produce a more complicatedand costly multilayer paper or board, e.g. coated paper or base paperwith an ink receiving layer.

The surface size materials used in the instant invention are generallysynthetic, preferably, though not necessarily, water soluble, and areprimarily hydrophobic, preferably containing at least about 30 molepercent hydrophobic groups, while the acrylamide polymers with whichthey are combined are substantially hydrophilic, preferably containingat least about 25 mole percent, more preferably at least about 50 molepercent hydrophilic monomer groups such as the amide group inpolyacylamide. The surface size and polyacrylamide material must bepremixed prior to application to the paper. The polyacrylamide surfacesize mixtures of the instant invention may be added alone as a watersolution in dispersion or in combination with commonly applied paperadditives such as size press starches. The compositions of the presentinvention may be used in the production of board such as boxboard,linerboard and bleached board, e.g. milk carton, as well as varioustypes of paper such as, for example uncoated, woodfine papers,packaging, newsprint, and ledger. The paper and board of the instantinvention is especially useful for paper or board which is designed tobe or capable of being printed in its end use, i.e. printing paper orboard.

In yet a further embodiment of the present invention compositionscomprising a hydrophobic surface size agent, a hydrophilicpolyacrylamide and a cross-linking agent, such as, for example, ammoniumzirconium carbonate, are provided and when applied to paper or board, atthe size press, or the like, further improve printability, bothnon-impact and impact printability, internal dry strength, and surfacestrength, e.g. tensile strength, burst, and wax pick.

SUMMARY OF INVENTION

In a broad sense, the instant invention provides compositions comprisingsubstantially hydrophobic surface size and a substantially hydrophilicmaterial such as polyacrylamide which may be used in paper or boardmaking to impart improved sizing, internal bond strength and printcharacteristics. The instant invention provides compositions, preferablyaqueous compositions, comprising a mixture, preferably a dispersion, ofsynthetic preferably carboxylated surface size containing at least about40 mole percent hydrophobic groups and water-soluble polyacrylamide,wherein the ratio of said surface size to said polyacrylamide is fromabout 30-95 parts by weight surface size to about 70-5 parts by weightpolyacrylamide. Preferred surface size agents include hydrolyzed styrenemaleic anhydride copolymers, styrene maleic acid salt copolymers,styrene maleic ester copolymers, styrene (meth)acrylate copolymers,styrene (meth)acrylate ester copolymers, styrene acrylate esteracrylonitrile terpolymers, acrylonitrile (meth)acrylate salt copolymersand polyurethanes. A further embodiment of the present inventionprovides compositions, preferably aqueous compositions, comprising amixture of synthetic, carboxylated polymeric surface size containing atleast about 40 mole percent hydrophobic groups and hydrophilicpolyacrylamide in the ratios described along with an effective amount ofstabilizing agent and\or crosslinking agent. Paper or board comprisingthe above-described compositions of the present invention are alsoprovided. In a preferred embodiment there is provided paper or board,preferably uncoated paper or board, comprising:

(i) synthetic, carboxylated surface size containing at least 40 molepercent hydrophobic groups and

(ii) substantially hydrophilic polyacrylamide, wherein the ratio of saidsurface size to said polyacrylamide is from about 30-95 parts by weightsurface size to about 70-5 parts by weight polyacrylamide.

Also, there is provided a method for improving the printability and/orstrength of printing paper or board, preferably uncoated paper or board,which comprises:

applying to the surface of paper or board, using a size press, calendar,or spray boom, a mixture comprising:

(i) synthetic surface size containing at least about 40 mole percenthydrophobic groups; and

(ii) substantially hydrophilic polyacrylamide;

wherein the ratio of said surface size to said polyacrylamide is fromabout 30-95 parts by weight surface size to about 70-5 parts by weightpolyacrylamide.

Also provided are methods for improving the printability and/or strengthof uncoated paper or board which comprise:

1) adding to a liquid medium in any order:

(a) a mixture comprising:

(i) synthetic, carboxylated polymeric surface size containing at least40 mole percent hydrophobic groups; and

(ii) substantially hydrophilic polyacrylamide;

wherein the ratio of said surface size to said polyacrylamide is frombout 30-95 parts by weight surface size to about 70-5 parts by weightpolyacrylamide; and

(b) an effective amount of crosslinking agent to form a dilute mixture;

wherein the weight ratio of liquid medium to said mixture ranges fromabout 70-99 parts liquid medium to about 30-1 parts mixture; and

2) applying said dilute mixture formed in step (1) to the surface ofpaper or board using a size press, calendar, or spray boom.

DETAILED DESCRIPTION OF THE INVENTION

The above and related objects of the present invention are achieved byproducing a mixture, preferably an aqueous mixture, preferably adispersion, comprising surface size containing at least about 30 molepercent, preferably at least about 40 mole percent hydrophobic groups,based on total moles in the sizing agent and a dry strength agent,generally a water-soluble or hydrophilic polyacrylamide. Surface sizematerials are any polymer that imparts sizing or liquid hold out.Generally surface size are non-ionic or anionic, preferably anionic.Suitable surface sizes are preferably polymeric, usually carboxylated,i.e., containing carboxyl groups, generally at least about 25 molepercent carboxyl groups, preferably at least about 50 mole percent, morepreferably at least about 60 mole percent carboxyl groups, based on thesizing agent. Any synthetic surface size may be used, includingnon-polymeric agents, e.g. alkylketene dimers as long as they imparthydrophobic character to the paper surface. Representative examples ofsuitable surface size polymers useful in the instant invention includecopolymers of styrene and maleic anhydride or copolymers derived fromstyrene maleic anhydride such as styrene maleic acid salts, hydrolyzedstyrene maleic anhydride, and styrene maleic ester copolymers, as wellas styrene (meth)acrylate copolymers, styrene (meth)acrylate estercopolymers, styrene (meth)acrylate ester acrylonitrile terpolymers,styrene (meth)acrylic acid copolymers, acrylonitrile (meth)acrylate saltcopolymers, polyurethanes, mixtures thereof and the like. Usually thesurface size materials contain at least about 40 mole percenthydrophobic groups, preferably at least about 50 mole percenthydrophobic groups, more preferably at least about 53 mole percenthydrophobic groups. Representative examples of hydrophobic groupsinclude subunits such as styrene and ∝-methyl styrene, mixtures thereofand the like. The surface size materials are preferably water-soluble.The molecular weight of the surface size polymers used in the presentinvention is not critical and generally ranges from at least about30,000 as determined by high pressure size exclusion chromatography.

The acrylamide polymers useful in the instant invention are generallywater-soluble and may be water-swellable. All (alk)acrylamide polymersare encompassed in the present invention such as acrylamide,methacrylamide, ethacrylamide and the like. The term polyacrylamide forpurposes of this invention includes all (alk)acrylamide homopolymers aswell as copolymers and functionalized polyacrylamides. Thepolyacrylamides may be anionic, cationic or nonionic, although,generally, cationic and anionic are more preferred. Various monomers,preferably ethylenically unsaturated monomers may be copolymerized with(alk)acrylamide monomers to form the polyacrylamides used in the instantinvention. Representative cationic monomers include dialkylammoniumchlorides, N,N-dialkylaminoalkyl(meth)acrylates,N,N-dialkylaminoalkyl(meth)acrylamides, salts, quaternaries and mixturesthereof. Anionic monomers useful in the practice of this invention maycomprise acrylic or methacrylic acid, furmaric acid, crotonic acid,maleic acid, salts thereof, 2-acrylamido-2-methylpropane sulfonic acid,styrene sulfonic acid and their salts and the like. Water-solublenonionic monomers suitable in the practice of this invention, generallycomprise N-vinyl pyrrolidone, N,N-dialylmethacrylamides, hydroxyalkylmethacrylates; N-vinylformamide and the like. Small quantities of othercopolymerizable monomers, such as methyl acrylate; methyl methacrylate,acrylonitrile; vinyl acetate; styrene etc, may also be used.

Generally anionic and nonionic comonomers may be included in theacrylamide copolymers in amounts up to about 20 mole percent, preferablyabout 10 mole percent, based on the copolymer. Cationic comonomers maybe included in the acrylamide copolymers in amounts up to about 15 molepercent, preferably up to about 10 mole percent, based on the copolymer.Preferable polyacrylamides include acrylamide homopolymers, copolymersof acrylamide which preferably contain from about 1 to about 20 molepercent anionic or cationic comonomers as well as hydrolyzedpolyacrylamide. Representative examples of preferred polyacrylamidesinclude copolymers of acrylamide with acrylate esters, acrylate salts,including sodium, potassium and ammonium salts, acrylic acid,diallyldimethylammonium chloride and the like. The various types ofpolyacrylamides may be used alone or in combinations of two or more.Copolymers of acrylamide and sodium acrylate are generally mostpreferred, more preferably those containing from about 1 to about 20acrylate salt, most preferably about 5 to about 10 acrylate salt. It isdesirable that the polyacrylamides used herein be substantiallyhydrophilic, generally containing at least about 30 mole percenthydrophilic groups, preferably at least about 50 mole percenthydrophilic groups, based on polyacrylamide, more preferably at leastabout 75 mole percent hydrophilic groups, e.g., non-akylated acrylamide.The molecular weight of polyacrylamides used herein is not critical,though a molecular weight as high as possible is usually preferred,typically at least about 50,000, preferably at least about 300,000 andpolyacrylamides typically can have a molecular weight up to about500,000, more preferably up to about 100,000. Polyacrylamides may becrosslinked with crosslinking agents known in the art such as, forexample those agents described hereinafter.

It is important in providing the benefits of the instant invention thatthe polyacrylamide and surface size is premixed before application tothe paper or board. Polyacrylamides are typically available in aqueoussolution form and it is preferred, in preparing the compositions of theinstant invention, to adjust the pH of the aqueouspolyacrylamide-containing solution to an alkaline pH, generally a pH ofat least about 8 preferably at least about 9, prior to admixing it withthe surface size. Preferably the pH of the aqueous solution ofpolyacrylamide is adjusted to a pH ranging from about 8 to about 11,more preferably about 9 to about 10. It is most desirable to adjust thepH of the aqueous solution of polyacrylamide to a pH that issubstantially the same as that of the surface size, which also typicallyis available in solution, preferably aqueous, form. Generally this is apH that is within about 1 pH unit. While it is not essential to combinethe polyacrylamide and surface size as aqueous solutions, it ispreferred. Similarly, it is not essential to adjust the pH of theaforesaid polymer solutions so that they are substantially similar, butit is preferred. When the pH of the polyacrylamide solution issubstantially different from that of the aqueous solution of surfacesize, gel formation or agglomeration tends to occur.

While the pH of the surface size/acrylamide polymer-containingcompositions of the present invention is not critical to performance, pHappears to affect physical properties of the composition; it is,therefore, generally preferred that the pH of the composition is atleast about 8 or above about 8 preferably at least about 9 and up toabout 11, preferably up to about 10. By adjusting the pH of the finalcomposition within this range, the composition is less viscous, easierto handle and more stable. Thus the compositions of the presentinvention preferably have a pH ranging from about 8 to about 11, morepreferably from about 9 to about 10; pH adjustments may be made prior toadding the composition to the paper or board but is preferably doneafter or during preparation of the composition. Typically any bases (oracids if the case may be) which are compatible with the particularsurface size and acrylamide polymers may be used for pH adjustments,preferably hydroxide, carbonate, bicarbonate bases such as, for example,sodium hydroxide, sodium bicarbonate, sodium carbonate, lithiumhydroxides, potassium hydroxide mixtures thereof and the like.

Generally the amount of polyacrylamide to be combined with surface sizeis not critical. Usually surface size and polyacrylamide are admixed ina ratio of from about 30-95 parts by weight surface size to about 70-5parts by weight polyacrylamide, preferably from about 40-70 parts byweight surface size to about 60-30 parts by weight polyacrylamide, morepreferably from about 50-60 parts by weight surface size to about 50-40parts by weight polyacrylamide. While mixing the materials within theabove-described ratios tends to provide a homogenous mixture which tendsto not exhibit phase separation, the ratios described herein are notcritical and may vary depending on the paper type, the propertiesdesired and additional materials added to the aqueous composition; theoptimal ratios for each papermaking system may be ascertained usingsimple experimentation. In mixing the surface size agent withpolyacrylamide to prepare the compositions of this invention, the orderof addition is not critical, though it is preferred to add the surfacesize to the polyacrylamide. The compositions can contain about 100percent polymer or surface size which comprise most, if not all of thesolids, though the compositions are preferably aqueous. The solidscontent of the aqueous composition should be at least about 0.5 weightpercent, based on total weight of the composition and broadly rangesfrom about 0.5 to about 30 percent, by weight, preferably about 10 toabout 22 percent, by weight, based on total weight of the composition.It is more preferred to dilute the surface size acrylamidepolymer-containing composition with water to obtain at least about 12percent solids, generally from about 12 to about 17 percent solids,preferably from about 13 to about 15 percent solids, by weight, based ontotal weight of the composition.

Generally, the compositions of the instant invention consist essentiallyof the above-described surface size and polyacrylamide, preferably thecompositions consist of the previously discussed surface size andpolyacrylamide in the ratios described above. Optionally, the surfacesize\polyacrylamide containing compositions preferably further containan effective amount of stabilizing agent sufficient to reduce phaseseparation. Any agent that reduces phase separation is suitable as astabilizing agent. Generally, cationic polyacrylamides, cationichomopolymers, such as polydiallyidimethyl ammonium chloride,polyhydroxyalkylamines and starch are preferred. Anionic polymers suchas, for example, carboxymethylcellulose and phophoralated starches arealso suitable stabilizing agents. Stabilizing agents may generally beadded in amounts sufficient to prevent phase seperation, generally atleast about 0.3 percent, preferably at least about 0.5 percent byweight, based on the weight of surface size and acrylamide polymer andup to about 5.0 percent, preferably up to about 3.0 percent by weight,based on the weight of surface size and acrylamide polymer. Accordingly,preferred amounts of stabilizing agent range from about 0.3 percent toabout 5.0 percent, based on the total weight of surface size andpolyacrylamide.

The paper and board that is produced acording to the instant inventionmay contain auxiliary materials known in the art as useful forincorporation into paper or board by adding them to the pulp at the wetend, directly to the paper or board or to a liquid medium, e.g. starchsolution which is then used to impregnate paper sheets or board.Representative examples of auxiliary agents include defoamers,bacteriocides, pigments, fillers and the like. The addition of acrosslinking agent, generally a water soluble crosslinking agent isparticularly preferred. It has been found that a crosslinking agenttends to enhance the improved sizing, strength and printability benefitsprovided by the surface size/polyacrylamide compositions. Generally anyknown crosslinking agent is useful in the instant invention, includingthose zirconium hafnium or titanium salts described in U.S. Pat. No.5,362,573 which is incorporated herein by reference, glyoxlated polymerssuch as glyoxylated vinylamide polymers, formaldehyde,melamine-formaldehyde condensation polymers and glyoxyl extendedmaterials such as dihydroxyethylene urea. Ammonium zirconium carbonate(AZC) is particularly preferred. The crosslinking agent may be addeddirectly to the composition or mixture containing surface size andacrylamide polymer or, applied to paper or board during the paper/boardmaking process, e.g., added directly to the paper or through use of aliquid medium such as water or aqueous starch solution at the sizepress. Crosslinking agent may be added to the liquid medium before,after or simultaneously with the polyacrylamide/surface size mixture. Aneffective amount of crosslinking agent sufficient to provide improvedstrength, sizing or printability properties should be used, generally anamount of at least about 0.3 percent, preferably about 2.0 percent, byweight, based on total weight of surface size and acrylamide polymer;and generally no more than about 10.0 percent, preferably no more thanabout 5.0 percent, by weight, based on total surface size and acrylamidepolymer; thus the range of crosslinking agent is generally from about0.3 to about 10.0 percent, by weight, based on total surface size andacrylamide polymer, preferably from about 2.0 to about 5.0 percent byweight, based on total surface size and acrylamide polymer. Whencrosslinker is added to a liquid medium such as an aqueous starchsolution or water, the amount of crosslinking agent used may be based onstarch pickup or water pick-up; typically amounts of crosslinker appliedto paper or board are those known in the art and preferably the amountranges from about 0.02 to about 10 pounds of crosslinking agent per tonof paper or paperboard, though this range may vary.

Generally, surface treatment may be applied to paper as either a postproduction operation or as a portion of the paper-making process itself.Surface size is applied typically in the papermaking process after thepaper sheet has been formed and dried but not passed through a calendarstack. The formed and dried sheet (web) is conducted through a sizepress (actually a blade, rod or roll coater of various configurations)which re-wets the sheet to some degree (depending on the type of sizepress) with a liquid medium such as starch solution or a starch/surfacesize solution, or a water/surface size solution. Upon the wetting of theweb, it is again dried and subsequently passed through a series of nipsat the calendar to control caliper and smoothness in the finished sheetprior to wind-up, slitting or sheeting.

In yet another embodiment of the instant invention, methods forimproving the printability and/or strength of paper or board, generallyprinting paper or board, are provided by applying to the surface ofpaper or board, the mixtures of the above described surface sizecompounds and substantially hydrophilic polyacrylamide in the ratiosdescribed above. The polyacrylamide/surface size containing compositionsare generally applied to base sheets or pre-formed paper or board (orwebs) at the dry end of the papermaking process using a size press,calendar spray boom, or the like. The size press is most common;however, the calendar, or calendar stack has been used with paperboard.Any size press designs may be used, including but not limited tohorizontal press, vertical press, gate roll size press and meteringblade size press, rod, puddle type, or combinations thereof. Thecompositions may also be sprayed onto the paper or board surface,optionally with a crosslinking or other auxiliary material. It isimportant that the compositions of the instant invention are added afterthe paper or board is formed in the papermaking process, for example atthe size press. When applied to paper or board using a liquid medium,such as starch solution or water, the polyacrylamide\surface sizemixture amount to be used generally depends on starch pick-up or waterpick-up, and may vary though preferably amounts sufficient to providefrom about 0.5 to about 15 pounds of polyacrylamide\surface size per tonof paper or board, preferably from about 1 to about 10 poundspolyacrylamide\surface size per ton of paper or board. Depending on themeans used for applying the surface size\polyacrylamide mixtures topaper or board (e.g., size press or calendar) and design of such, thecompositions or mixtures added to the paper or board may be diluted toabout 0.5 to about 6 percent solids. Optionally, thepolyacrylamide\surface size containing compositions of the presentinvention may, prior to addition to the paper or board, be admixed witha liquid medium, such as water. The surface size/polyacrylamidecompositions are added to liquid medium, typically contained in a sizepress or the like, to form a dilute mixture and paper sheets orpre-formed board is contacted with the dilute mixture or liquid mediumcontaining surface size/polyacrylamide and optionally crosslinkingagent. Preferably the liquid medium is water, more preferably an aqueousstarch solution. Any type of starch may be used including cationicnonionic or anionic starch. Typical starch solutions range inconcentration from about 3 to about 13 weight percent, preferably fromabout 6 to about 8 weight percent starch solids; however, theseconcentrations may vary depending on the type of size press. The ratioof liquid medium to polyacrylamide\surface size mixture may also varydepending on the paper grade and type of size press and type of liquidand the sheet properties that are desired. Generally, however, theweight ratio of liquid medium to mixture comprisingpolyacrylamide\surface size ranges from about 70-99 parts liquid mediumto about 30-1 part mixture comprising polyacrylamide/surface size,preferably from about 90-99 parts by weight liquid medium to about 10-1part by weight mixture. Preferably, the weight ratio of aqueous starchsolution (liquid medium) to surface size/polyacrylamide containingcomposition is from about 95-99 parts starch solution to about 5-1 partsmixture comprising surface size/polyacrylamide. If water is used as theliquid medium, preferred ratios are about 90-95 parts water to about10-5 parts surface size/polyacrylamide containing composition. After orbefore adding the polyacrylamide\surface size mixture to a liquid mediumsuch as water or aqueous starch solution to form a dilute mixture, itmay be preferable to adjust the pH of the dilute mixture to from about 6to about 11, preferably from about 7 to about 10, prior to contactingthe dilute mixture with the pre-formed paper or board. The pH of thedilute mixture may also be above about 8 to about 11.

After application of the polyacrylamide\surface size mixtures or dilutemixtures (further containing water or starch solution) to the paper orboard, the paper or board is typically dried using drying means known inthe art such as, for example, steam heated dryer cans, and infraredheaters. Any suitable drying temperature may be implemented andgenerally temperatures ranging from about 220° F. to about 260° F.,preferably from about 235° F. to about 255° F. are used.

Also provided by the instant invention are paper and board madeaccording to the methods described above as well as paper and boardwhich comprises the polyacrylamide\surface size compositions or mixturesdescribed above including those which further contain an effectiveamount of crosslinking agent and\or stabilizing agent as describedabove. The paper and board comprises substantially near the surface,surface size comprising at least about 30, preferably at least about 40mole percent hydrophobic groups and substantially hydrophilicpolyacrylamide generally in a ratio of from about 30-95 parts by weightsurface size to about 70-5 parts by weight polyacrylamide. As a resultof addition at the dry end, the surface size and polyacrylamide aregenerally localized substantially on the paper or board surface asopposed to being uniformly distributed throughout the paper or board .The paper and board of the instant invention are especially designed forprinting and are preferably uncoated. Paper and board of the instantinvention may be acid, alkaline or neutral paper, through preferablyalkaline. Preferably amounts of polyacrylamide\surface size range fromabout 0.5 to about 15 pounds of polyacrylamide\surface size per ton ofpaper or board, preferably from about 1 to about 10 poundspolyacrylamide\surface size per ton of paper or board. The invention isfurther illustrated by the following examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preparation of Liquid Media(aqueous starch solution)

To a stainless steel beaker, 233 grams (herein "parts") of a typicalsize press starch (containing about 0.17 mole percent cationic groups)was slurried in 767 parts of water with good agitation to from anaqueous starch slurry containing about 20 percent by weight starch. Thetemperature of the resulting slurry was then adjusted with steam toabout 90°-95° C. Once steady state was achieved, the temperature wasmaintained for about 30 minutes. After that time, the heat was removedfrom the starch cooker and the resulting starch was placed in anice-water bath. The temperature of the resulting solution was reduced toless than about 40° C. The starch solids were then adjusted to a finalconcentration of about 3 percent, by weight, with water. Allapplications or tubsizing herein (unless noted otherwise) were performedusing 3 weight percent cooked starch solution.

EXAMPLE 1

A suitable vessel was fitted with a stirring apparatus, thermometer, anitrogen inlet, and a water cooled reflux condenser and then placed in aheating bath. Addition ports to provide monomer feeds were fitted to theflask using a rubber septum.

The reaction vessel was charged with the requisite amount of water toeffect a total solids of about 15 percent, by weight. The vessel wasthen heated to reflux at which time the monomer feed consisting ofacrylamide (90 parts by weight) and acrylic acid (10 parts by weight)was fed into the refluxed reactor over a period of ninety minutes.Concurrently, a catalyst system was simultaneously added to the reactorto effect polymerization. After addition was completed, the temperaturewas maintained at reflux for an additional 30 minutes. The reactor wasthen cooled to ambient temperature. The resulting polymer containedabout 10 weight percent acrylic acid or about 8 mole percent acrylicacid and had a solution pH of between about 4.5-5.5.

A styrene/acrylic acid copolymer was prepared according to the knownprocedures disclosed in U.S. Pat. No. 5,138,004, which is incorporatedherein by reference. The copolymer contained about 50 mole percentstyrene and about 50 mole percent acrylic acid. Approximately 50 molepercent hydrophobic groups were present in the styrene/acrylic acidcopolymer. The surface size/polyacrylamide composition of the instantinvention was prepared by adding the above polyacrylamide solution tothe styrene/acrylic copolymer. The weight ratios of styrene/acrylicsurface size to polyacrylamide was about 55 parts styrene/acrylic toabout 45 parts by weight polyacrylamide. Water was added to adjust thepolymer solids to about 14 percent, by weight, total solids and the pHwas adjusted to about 9.5.

EXAMPLE 2

A starch solution containing the surface size polyacrylamide mixtureprepared in Example 1 was applied to base sheet paper containing nosurface additives according to the following procedure.

Application Procedure:

600 parts of the above 3 percent starch solution was weighed into astainless steel rectangular pan. An 8.5×11 inch sheet of alkaline basepaper is then drawn through the starch solution at an even speed todeposit starch on the surfaces (top and bottom) of the base sheet paper.The sheet weighed about 3.9 parts. The sheet is then blotted usingwaterleaf (blotter) paper and the weight of the blotted sheet is thenrecorded. The pickup weight of the starch-treated sheet is calculated bytaking the difference between the weight of the dry sheet and the wetsheet. The starch pickup weight was 2.3 parts by weight. Pick-up weightfor the blank was then used to calculate the amount of resin needed tobe added for the rest of the testing. 5.5 parts of 24 weight percentsolution of the surface sizing polyacrylamide mixture was added to thestarch solution to provide about 1.0 part by weight surfacesize/polyacrylamide to the starch solution. The sheet was then pressedon a Noble and Wood press and drum dried at a temperature of about 245°F. for 30 seconds. All measurements of strength and sizing wereperformed after conditioning the paper for 24 hours at constanttemperature and humidity.

The sheets were then tested for strength using a Scott bond tester,TAPPI procedure #541, which is incorporated herein by reference. Theinternal bond of the paper averaged 138 mil-ft-lbs.

EXAMPLE 3

A commercially available sodium salt of styrene maleic acid copolymer("SMA") which is made from styrene and maleic anhydride comonomers wasobtained which contained approximately 50 mole percent hydrophobicgroups. An acrylamide/acrylic acid copolymer prepared as in Example 1was added to the SMA; the ratio of SMA to polyacrylamide was 65 parts byweight SMA to 35 parts by weight polyacrylamide. The procedure inExample 1 was followed except that the polymer solids were adjusted toabout 15 percent by weight and the pH of the mixture was adjusted toabout 9.5.

EXAMPLE 4

To 600 parts of the above described 3 percent by weight aqueous starchsolution, 5.5 parts of a 24 percent solution (1.0 part by weight active)of the SMA/polyacrylamide mixture prepared in Example 3 was added. Thestarch pick-up was about 2.3 parts by weight. The aforesaid mixture wasthen applied to basesheet paper according to the application procedurein Example 2. The sheets were then tested for strength using a Scottbond tester, TAPPI procedure #541. The internal bond of the paperaveraged 155 mil-ft-lbs.

EXAMPLE 5

Following the procedure of Example 2, various surface size agents wereadded to 600 parts of 3 percent by weight aqueous starch solutionsamples and the aforesaid was applied to basesheet paper according tothe application procedure described above. The starch pick-up was about2.3 parts by weight. The amount of surface size added was 1.0 part byweight. Internal bond strength of the resulting paper samples wasmeasured and appears in Table 1 along with internal bond strengthmeasurements for paper sized with the polyacrylamide surface sizingmixtures of the instant invention. The results demonstrate that use ofsurface size/polyacrylamide provides significantly improved strengthcompared to the use of surface size agents alone.

                  TABLE 1                                                         ______________________________________                                                         InternaI Bond                                                Paper Additive   (mil-ft-lbs)                                                 ______________________________________                                        *SMA             115                                                          *Styrene Acrylic 122                                                          *Styrene Acrylate salt Na                                                                      118                                                          Example 3        155                                                          Example 1        138                                                          ______________________________________                                         *COMPARATIVE                                                             

EXAMPLE 6

The procedures in Examples 1 and 2 were followed to produce paperimpregnated with a styrene/acrylic acid polyacrylamide mixture of theinstant invention, except that 5.6 parts by weight of Ammonium ZirconiumCarbonate (AZC) was added to the starch solution. The surfacesize/polyarcylamide was added to the starch solution first followed bythe AZC. The starch pickup was 2.3 parts by weight. The internal bondstrength of the resulting paper was 182 mil-ft-lbs.

EXAMPLE 7

The procedures in Examples 3 and 4 were followed to produce paper withSMA/polyacrylamide mixture of the instant invention, except that 5.6parts of AZC was added to the starch solution after the addition of theSMA/polyacrylamide. The pickup was 2.3 parts by weight. The internalbond strength of the resulting paper was 195 mil-ft-lb.

EXAMPLE 8

Following the procedure in Example 5, SMA, styrene acrylic acidcopolymer and styrene acrylate salt surface size agents were added inamounts of 1.0 part, by weight surface size to 600 parts of 3 percentaqueous starch solution, except that 5.6 parts by weight of AZC wasadded to the starch solution after the addition of the surface size. Thedilute mixture was applied to the paper according to the aboveapplication procedure and the internal bond measured using a Scott bondtester. Internal bond measurements appear in Table 2, along withinternal bond measurements for mixtures prepared in Examples 6 and 7.The results demonstrate that a crosslinker significantly enhances theimproved bond strength produced by the surface size/polyacrylamidemixtures of the instant invention.

                  TABLE 2                                                         ______________________________________                                        Paper Additive   InternaI Bond                                                ______________________________________                                        *SMA             118                                                          *Styrene Acrylic 122                                                          *Styrene Acrylate sait Na                                                                      122                                                          Example 7        195                                                          Example 6        182                                                          ______________________________________                                         *COMPARATIVE EXAMPLES                                                    

EXAMPLES 9-10

The general procedure in Example 2 was followed for incorporatingvarious surface sizing or dry strength resins into paper samples usedfor comparative testing. For Examples 9 and 10 two surface sizepolyacrylamide mixtures were prepared following the procedures inExamples 3 and 1 respectively. Example 9 contained 65 parts SMA and 35parts by weight acrylamide acrylic acid copolymer. Example 10 contained55 parts styrene acrylic and 45 parts by weight acrylamide acrylic acidcopolymer. The acrylamide/acrylic acid copolymer used in both exampleswas prepared as in Example 1 and contained about 8 to about 10 molepercent acrylic acid. In certain instances 5.6 parts of AZC was addedafter addition of the surface size, polyacrylamide or surface sizepolyacrylamide mixture. Wax pick, an indicator of surface strength(higher number indicating more surface pick resistance), was measuredfor each paper sample as in TAPPI test procedure: T 459 which isincorporated herein by reference. The data is set forth in Table 3.

                  TABLE 3                                                         ______________________________________                                        PAPER ADDITIVE   AZC(Yes/No)                                                                              Wax Pick #                                        ______________________________________                                        *No Sizing or Dry strength                                                                     Yes        13                                                *Styrene Acrylate salt Na                                                                      No         12                                                *Styrene Acrylate salt Na                                                                      Yes        16                                                Polyacrlyamide   No         14                                                *Polyacrylamide  Yes        16                                                Example 9        No         12                                                Example 9        Yes        16                                                Example 10       No         16                                                Example 10       Yes        18                                                ______________________________________                                         *COMPARATIVE                                                             

EXAMPLES 11-12

For Examples 11 and 12 two surface size/polyacrylamide compositions wereprepared following Examples 3 and 1 and were incorporated into paperaccording to the procedure in Examples 4 and 2 respectively. Example 11contained 65 parts SMA and 35 parts by weight of acrylamide/acrylic acidcopolymer. Example 12 contained 55 parts styrene acrylic and 45 parts byweight acrylamide/acrylic acid copolymer. The acrylamide/acrylic acidcopolymer was prepared according to the procedure in Example 1 andcontained about 8 mole percent acrylic acid. Styrene acrylate sodiumsalt ("SA"), polyurethane and alkyl ketene dimer ("AKD") surface sizeswere each incorporated into paper samples by adding an amount of 1.0part by weight to the liquid medium following the procedure in Example2.

The resulting paper samples were then printed using non-impact printingtechniques (Hewlett Packard DeskJet 500C printer). The sheets wereprinted in both monochrome and color ink using the Hewlett Packard PaperAcceptance criteria for HP DeskJet 500L, 500C, 560C printers. Theprinted sheets were then tested for various print quality parameters.Optical density, dry time and wicking test criteria were measuredaccording to the Hewlett Packard paper acceptance booklet. Color bleedof the sheet was measured using the Olympus Cue-2 Morphometry program.This test measured the percent of a specific black letter on the colortest sheet on a yellow background. The bleed through of the paper wasquantitatively analyzed by looking at the back-side of the 100 percentrectangle on the Monochrome test sheet. A number from 1 to 5 wasassigned for each of the tests with 1 considered as poor and 5considered as excellent. A sheet given a number 3 for any test wasconsidered average. Table 4 summarizes the various print qualityparameters for each paper sample.

                  TABLE 4                                                         ______________________________________                                                                        *Poly-                                               Example 11                                                                            Example 12                                                                              *SA    urethane                                                                             *AKD                                   ______________________________________                                        Dry time 4         5         3    4      1                                    Bleedthrough                                                                           3         5         2    3      5                                    Optical Density                                                                        5         5         5    5      4                                    Color Bleed                                                                            4         5         5    3      2                                    ______________________________________                                         *COMPARATIVE                                                             

EXAMPLE 13

The procedure in Example 2 was used to impregnate into paper samplesvarious surface size materials as well as surface size/polyacrylamidecompositions prepared according to Examples 1 and 3. The amounts ofsurface size/polyacrylamide mixture, styrene acrylic acid, polyurethaneand alkene ketene dimer were about 1.0 part by weight. Paper sampleswere then tested for toner adhesion. The toner adhesion test measuresthe amount of reprographic toner that remains after distortion on acopied image. The test was performed using a Olympus Cue-2 Morphometryprogram that qualitatively measures the white area made by folding andcreasing the copied image. A number from 1 to 5 was assigned for each ofthe tests, with 1 considered poor and 5 considered excellent. A sheetgiven a number 3 for any test was considered average. Table 5 summarizesthe results and demonstrates that the surface size/polyacrylamide sizingagents of this invention provide improved toner adhesion.

                  TABLE 5                                                         ______________________________________                                        Paper Additive  Toner Adhesion                                                ______________________________________                                        Example 3       5                                                             Example 1       5                                                             *Styrene Acrylic                                                                              5                                                             *Polyurethane   2                                                             *Alkene ketene dimer                                                                          2                                                             ______________________________________                                         *COMPARATIVE                                                             

EXAMPLE 14

The procedure in Examples 1 and 2 were followed to produce paperimpregnated with a composition containing styrene acrylic copolymer andpolyacrylamide, except that 600 parts of water was used as the liquidmedium instead of the 3 percent starch solution. Four different dosagesof the styrene acrylic/polyacrylamide were added to the water. Themixture contained 55 parts of styrene acrylic and 45 parts by weightpolyarylamide/acrylic acid copolymer. Sizing was measured using a waterabsorbency drop test according to the following general procedure: water(pH 7) was applied to paper using a micro syringe (5.0 microliters) toform one drop on the paper. The time period in which the water dropcompletely absorbed into the paper was measured in seconds. Table 6summarizes the results, which demonstrate that the surfacesize/polyacrylamide compositions of the instant invention provideimproved sizing, particularly when compared to the data in Table 7below.

                  TABLE 6                                                         ______________________________________                                        Dosage (parts polymer)                                                                         Water drop (secs)                                            ______________________________________                                        0.2              32                                                           0.4              29                                                           0.8              40                                                           1.4              57                                                           2.8              70                                                           ______________________________________                                    

EXAMPLE 15

Example 14 was repeated except styrene acrylic copolymer and apolyacrylamide were individually applied to separate paper samples,instead of the styrene acrylic/polyacrylamide mixture. Water drop testdata was obtained as described in Example 14 and the results whichappear in Table 7 demonstrate that the use of the surfacesize/polyacrylamide mixtures of the instant invention (as shown in Table6) provide superior sizing compared to the individual components.

                  TABLE 7                                                         ______________________________________                                        Dosage    Water Drop Penetration                                                                       Water Drop Penetration                               (parts polymer)                                                                         Styrene acrylic (secs)                                                                       Polyacrylamide (secs)                                ______________________________________                                        0.2       29             21                                                   0.4       24             26                                                   0.8       33             32                                                   1.4       34             28                                                   2.8       38             35                                                   ______________________________________                                    

All of the above mentioned patents and publications are incorporatedherein by reference. Variations of the present invention will suggestthemselves to those skilled in this art in light of the above detaileddescription. Variations and modifications to the compositions andmethods of the instant invention can be made by one skilled in the artwithout departing from the spirit or scope of the invention as definedin the claims set forth below.

We claim:
 1. A method for improving the printability or strength ofprinting paper or board which comprises:applying to the surface of paperor board a mixture comprising:(i) a synthetic, polymeric surface sizecontaining at least about 40 mole percent hydrophobic groups; and (ii) asubstantially hydrophilic polyacrylamide;wherein the ratio of saidsurface size to said polyacrylamide is from about 30-95 parts by weightsurface size to about 70-5 parts by weight polyacrylamide based on 100parts by weight combined total surface size and polyacrylamide, saidmixture applied in an amount effective to improve one or both of theprintability or strength of the paper or board.
 2. A method according toclaim 1 wherein said surface size is selected from the group consistingof: hydrolyzed styrene maleic anhydride copolymers, styrene maleic acidsalt copolymers, styrene maleic ester copolymers, styrene (meth)acrylatecopolymers, styrene (meth)acrylate ester copolymers, styrene(meth)acrylate ester acrylonitrile terpolymers, alkylketenediamers,styrene(meth)acrylic acid copolymers, acrylonitrile (meth)acrylate saltcopolymers and polyurethanes.
 3. A method according to claim 1 whereinsaid polyacrylamide is selected from the group consisting of copolymersof acrylamide and acrylate salt, copolymers of acrylamide and acrylicacid, acrylamide homopolymers, crosslinked polyacrylamide and copolymersof acrylamide and diallyidimethyl ammonium chloride.
 4. A methodaccording to claim 1 wherein said polyacrylamide has a molecular weightof at least about 50,000.
 5. A method according to claim 1 wherein aneffective amount of a crosslinking agent is added to said paper orboard.
 6. A method according to claim 5 wherein said crosslinking agentis selected from the group consisting of zirconium salts, hafnium salts,titanium salts, dihydroxyethylene urea, formaldehyde, glyoxylatedvinylamide polymers and melamine-formaldehyde condensation polymers. 7.A method according to claim 1 wherein said mixture further comprises aneffective amount of a stabilizing agent, wherein the amount of saidstabilizing agent is from about 0.3 percent to about 5 percent, byweight, based on the total weight of surface size and polyacrylamide. 8.A method according to claim 7 wherein said stabilizing agent is cationicstarch, polyhydroxylalkylamines, polydiallyldimethylammonium chloride,carboxymethyl cellulose, cationic polyacrylamides or phosphoralatedstarches.
 9. A method according to claim 1 wherein said mixture isapplied to said paper or board by adding said mixture to a liquid mediumto form a dilute mixture and then contacting said paper or board withsaid dilute mixture, the weight ratio of liquid medium to said mixtureis about 70-99 parts by weight liquid medium to about 30-1 part byweight mixture.
 10. A method according to claim 9 wherein said liquidmedium is water or an aqueous starch solution.
 11. A method according toclaim 9 which further comprises adjusting the pH of said liquid mediumto a pH ranging from about 6 to about 11 prior to or after adding saidmixture to said liquid medium.
 12. A method according to claim 1 whereinsaid mixture is applied to paper or board using a size press.
 13. Amethod according to claim 1 wherein the amount of said mixture appliedto said paper or board is an amount sufficient to provide from about 0.5to about 15 pounds of total surface size and polyacrylamide per ton ofpaper or board.
 14. A method according to claim 1 wherein the percentageof solids in said mixture ranges from about 0.5 to about 30 weightpercent solids, based on the total weight of said mixture.
 15. A methodfor improving the printability or strength of paper or board whichcomprises:1) adding to a liquid medium in any order:(a) a mixturecomprising:(i) a synthetic, polymeric carboxylated surface sizecontaining at least about 40 mole percent hydrophobic groups; and (ii) asubstantially hydrophilic polyacrylamide;wherein the ratio of saidsurface size to said polyacrylamide is from about 30-95 parts by weightsurface size to about 70-5 parts by weight polyacrylamide based on 100parts by weight combined total surface size and polyacrylamide; and (b)an effective amount of crosslinking agent to form a dilutemixture;wherein the weight ratio of liquid medium to said mixture rangesfrom about 70-99 parts liquid medium to about 30-1 parts mixture basedon 100 parts by weight combined total liquid medium and mixture; and 2)applying said dilute mixture formed in step (1) to the surface of paperor board.
 16. A method according to claim 15 wherein said liquid mediumis water or aqueous starch solution.
 17. A method for improving theprintability or strength of uncoated paper or board which comprises:(1)forming a dilute mixture by adding to an aqueous starch solution in anyorder:(a) a mixture comprising:(i) a surface size selected from thegroup consisting of: hydrolyzed styrene maleic anhydride copolymers,styrene maleic acid salt copolymers, styrene maleic ester copolymers,styrene (meth)acrylate copolymers, styrene (meth)acrylate estercopolymers, styrene (meth)acrylate ester acrylonitrile terpolymers,styrene (meth)acrylic acid copolymers, acrylonitrile (meth)acrylate saltcopolymers, and polyurethanes; and (ii) a substantially hydrophilicpolyacrylamide;wherein the ratio of said surface size to saidpolyacrylamide is from about 40-70 parts by weight surface size to about60-30 parts by weight polyacrylamide based on 100 parts by weightcombined total surface size and polyacrylamide, and said mixturecomprises from about 0.5 to about 30 weight percent solids; and (b) aneffective amount of a crosslinking agent; 2) applying said dilutemixture formed in step (1) to the surface of paper or board, in anamount sufficient to deposit from about 0.5 to about 15 pounds of totalsurface size and polyacrylamide per ton of paper or board.
 18. Themethod according to claim 1 wherein said synthetic surface size isnon-ionic or anionic.
 19. The method according to claim 18 wherein saidsynthetic surface size is anionic.
 20. The method according to claim 1wherein said synthetic surface size contains at least about 25 molepercent carboxylated groups.
 21. The method according to claim 20wherein said surface size is selected from the group consisting of:hydrolyzed styrene maleic anhydride copolymers, styrene maleic acid saltcopolymers, styrene maleic ester copolymers, styrene (meth)acrylatecopolymers, styrene (meth)acrylic acid copolymers, and acrylonitrile(meth)acrylate salt copolymers.
 22. The method according to claim 20whereina. said surface size is selected from the group consisting of:hydrolyzed styrene maleic anhydride copolymers, styrene maleic acid saltcopolymers, styrene maleic ester copolymers, styrene (meth)acrylatecopolymers, styrene (meth)acrylic acid copolymers, and acrylonitrile(meth)acrylate salt copolymer; and, wherein said mixture furthercomprises b. a stabilizing agent in an amount of from about 0.3 percentto about 5 percent by weight, based on total surface size andpolyacrylamide.
 23. The method according to claim 20 wherein saidsurface size is a styrene acrylic acid copolymer.
 24. The methodaccording to claim 23 wherein said surface size is a styrene acrylicacid copolymer.